Gene Autry plays a singing cowboy of the same name, who runs Radio Ranch, a dude ranch from which he makes a daily live radio broadcast at 2pm. This is a "modern" cowboy story, with planes and such. Gene has two kid sidekicks, Frankie Darro and Betsy King Ross, who lead a club, the "Junior Thunder Riders," in which the kids play at being armoured knights of an unknown civilization, the mysterious Thunder Riders who make a sound like thunder when they ride. The kids, dressing up in capes and water-bucket helmets, play at riding "to the rescue!" (to quote their motto).

A chance to be real heroes occurs when Betsy, Frankie and Gene are kidnapped by the real Thunder Riders, from the super-scientific underground empire of Murania, complete with towering skyscrapers, robots, ray-guns, elevators tubes that extend miles from the surface, and an icy, evil blonde Queen Tika. On the surface, a group of crooks under Prof Beetson plan to invade Murania and seize its radium wealth, while in Murania, a group of revolutionaries plot to overthrow Queen Tika.

The inhabitants of Murania are the lost tribe of Mu and went below the surface in the Ice Age, 100,000 years ago and now live in a fantastically advanced city 20 or 25,000 feet underground and cannot now breathe the air at ground level so must wear masks. Gene Autry however has no trouble breathing their air. The Thunder Guard (Riders) emerge into the surface world from a cave where a huge rock door opens upwards, remindful of Ali Baba. Both Muranians and Prof Beetson's team want to get rid of Autry so he loses his radio contract and Radio Ranch becomes vacant.